Circular dichroism measuring method and circular dichroism measuring device

ABSTRACT

A circular dichroism measuring method using a circular dichroism measuring device 2 includes a step of measuring Ip(t) (S101: phase amount change acquisition step), a step of measuring Is(t) (S102: sample data acquisition step), a step of converting Ip(t) to δ(t) (S103: phase amount change acquisition step), a step of converting Is(t) to Is(δ) (S104: analysis step/Is(δ) calculation step), and a step of performing curve fitting to calculate matrix elements S00, S02, and S03 (S105: analysis step/matrix element calculation step).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 15/315,433, filed Dec. 1, 2016, which is a § 371 ofInternational Application No. PCT/JP2015/064205, filed May 18, 2015,which claims the benefit of Japanese Patent Application No. 2014-114061,filed Jun. 2, 2014, the entire contents each of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a circular dichroism measuring methodand a circular dichroism measuring device.

BACKGROUND ART

Circular dichroism (CD) is a phenomenon caused by optical activity(chirality) of molecules and is defined as a difference in absorbancewith respect to left and right circularly polarized light. Sincespectral information of this circular dichroism reflects a high-orderstructure of molecules, circular dichroism is particularly suitablyapplied to, for example, analysis of a high order structure ofbiologically active substance. For this circular dichroism, a method ofirradiating a sample with left and right circularly polarized lights andobtaining a difference in absorbance from an intensity difference oftransmitted light is generally used.

In measurement of the circular dichroism, measurement using a so-calledmodulation method is generally used. In a modulation method, an opticalphase modulator such as a photoelastic modulator or a Pockel cell isused as a circular polarization modulator that generates circularlypolarized light. However, modulation of the circular polarizationmodulator in which there is a distortion component is known (see, forexample, Patent Literature 1 and Non Patent Literature 1).

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent No. 3341928

Non Patent Literature

-   [Non Patent Literature 1] Shinto, Spectroscopy Study, Vol. 34, No.    3, page 153 (1985)

SUMMARY OF INVENTION Technical Problem

In recent years, various studies have been performed in order tomanufacture a circular dichroism measuring device at a lower cost.However, an inexpensive phase modulation element in which there is adistortion component cannot be applied as a phase modulation element(circular polarization modulator) constituting the circular dichroismmeasuring device, and there is a problem of achievement of a low cost.

The present invention has been made in view of the above circumstances,and relates to a circular dichroism measuring method and a circulardichroism measuring device capable of accurately measuring circulardichroism even when a phase modulation element with a distortioncomponent is used.

Solution to Problem

To achieve the above object, a circular dichroism measuring methodaccording to an aspect of the present invention is a circular dichroismmeasuring method in a circular dichroism measuring device including alight source, a polarization plate that extracts linearly polarizedlight from light emitted from the light source, a phase modulationelement that modulates the linearly polarized light, and a lightdetector that detects light modulated by the phase modulation elementand then transmitted through a sample, the circular dichroism measuringmethod including: a sample data acquisition step of acquiring a changein a light intensity with respect to time in the light detector; a phaseamount change acquisition step of acquiring a change in a phase amountwith respect to time of the phase modulation element; and an analysisstep of converting the change in light intensity acquired in the sampledata acquisition step into a change with respect to the phase amount onthe basis of the change in the phase amount acquired in the phase amountchange acquisition step, and calculating matrix elements S00, S02, andS03 when a Mueller matrix according to the sample is as shown inEquation (1) below on the basis of the change.

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 1} \right\rbrack & \; \\{S = {{S(\theta)} = {e^{- {Av}} \cdot {\begin{pmatrix}{S\; 00} & {S\; 01} & {S\; 02} & {S\; 03} \\{S\; 10} & {S\; 11} & {S\; 12} & {S\; 13} \\{S\; 20} & {S\; 21} & {S\; 22} & {S\; 23} \\{S\; 30} & {S\; 31} & {S\; 32} & {S\; 33}\end{pmatrix}.}}}} & (1)\end{matrix}$

According to the circular dichroism measuring method, a temporal changein a phase amount in the phase modulation element is acquired, thechange in light intensity acquired in the sample data acquisition stepis converted into a change with respect to the phase amount on the basisof the change, and then, matrix elements in the Mueller matrix arecalculated. Since the matrix elements in the Mueller matrix arecalculated in consideration of the temporal change of the phase amountof the phase modulation element, when the phase modulation element has adistortion component, the matrix elements can be calculated inconsideration of this. Thus, it is possible to accurately measure thecircular dichroism.

Here, the phase amount change acquisition step includes acquiring dataIs(t) indicating a temporal change in a light signal detected by thelight detector by emitting light from the light source in a state inwhich the sample is disposed, and the phase amount change acquisitionstep includes: a step of acquiring data Ip(t) indicating a temporalchange in a light signal detected in the light detector by emittinglight from the light source in a state in which a second polarizationplate having a relationship with the polarization plate giving crossedNicols is disposed on an optical path from the light source, in place ofthe sample; and a step of converting the data Ip(t) into data δ(t)indicating a temporal change of the phase change amount due to the phasemodulation element using Equation (2) below,

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 2} \right\rbrack & \; \\{{\delta(t)} = {\cos^{- 1}\left( {1 - \frac{2{I_{P}(t)}}{I_{m}}} \right)}} & (2)\end{matrix}$and the analysis step includes an Is(δ) calculation step of calculatingdata Is(δ) according to the phase amount at time t of data Is(t) of thesample on the basis of the data δ(t) of the phase change amount; and amatrix element calculation step of performing fitting on the data Is(δ)using Equation (3) below to calculate matrix elements S00, S02, and S03in a Mueller matrix according to the sample.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack & \; \\{I = {\frac{1}{2}{e^{- {Av}} \cdot \left( {{S\; 00} + {S\; 0\; 3\mspace{11mu}\cos\;\delta} - {S\; 02\mspace{11mu}\sin\;\delta}} \right)}}} & (3)\end{matrix}$

As described above, it is possible to perform circular dichroismmeasurement in consideration of a temporal change of the phase changeamount by calculating the data Is(δ) according to the phase amount attime t of the data Is(t) of the sample on the basis of the data δ(t) ofthe phase change amount, and performing fitting on the data Is(δ) tocalculate matrix elements S00, S02, and S03 in a Mueller matrixaccording to the sample.

The invention of the above circular dichroism measuring method can bedescribed as an invention of a circular dichroism measuring device asfollows.

That is, a circular dichroism measuring device according to an aspect ofthe present invention is a circular dichroism measuring device includinga light source, a polarization plate that extracts linearly polarizedlight from light emitted from the light source, a phase modulationelement that modulates the linearly polarized light, and a lightdetector that detects light modulated by the phase modulation elementand then transmitted through a sample, the circular dichroism measuringdevice including: a sample data acquisition means for acquiring a changein a light intensity with respect to time in the light detector; a phaseamount change acquisition means for acquiring a change in a phase amountwith respect to time of the phase modulation element; and an analysismeans for converting the change in light intensity acquired in thesample data acquisition means into a change with respect to the phaseamount on the basis of the change in the phase amount acquired in thephase amount change acquisition means, and calculating matrix elementsS00, S02, and S03 when a Mueller matrix according to the sample is asshown in Equation (4) below on the basis of the change.

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 4} \right\rbrack & \; \\{S = {{S(\theta)} = {e^{- {Av}} \cdot {\begin{pmatrix}{S\; 00} & {S\; 01} & {S\; 02} & {S\; 03} \\{S\; 10} & {S\; 11} & {S\; 12} & {S\; 13} \\{S\; 20} & {S\; 21} & {S\; 22} & {S\; 23} \\{S\; 30} & {S\; 31} & {S\; 32} & {S\; 33}\end{pmatrix}.}}}} & (4)\end{matrix}$

Further, the phase amount change acquisition means acquires data Is(t)indicating a temporal change in a light signal detected in the lightdetector by emitting light from the light source in a state in which thesample is disposed, and the phase amount change acquisition meansincludes: an Ip(t) acquisition means for acquiring data Ip(t) indicatinga temporal change in a light signal detected in the light detector byemitting light from the light source in a state in which a secondpolarization plate having a relationship with the polarization plategiving crossed Nicols is disposed on an optical path from the lightsource, in place of the sample; and a conversion means for convertingthe data Ip(t) into data δ(t) indicating a temporal change of the phasechange amount due to the phase modulation element using Equation (5)below,

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 5} \right\rbrack & \; \\{{\delta(t)} = {\cos^{- 1}\left( {1 - \frac{2{I_{P}(t)}}{I_{m}}} \right)}} & (5)\end{matrix}$and the analysis means includes an Is(δ) calculation means forcalculating data Is(δ) according to the phase amount at time t of dataIs(t) of the sample on the basis of the data δ(t) of the phase changeamount; and a matrix element calculation means for performing fitting onthe data Is(δ) using Equation (6) below to calculate matrix elementsS00, S02, and S03 in a Mueller matrix according to the sample.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 6} \right\rbrack & \; \\{I = {\frac{1}{2}{e^{- {Av}} \cdot \left( {{S\; 00} + {S\; 0\; 3\mspace{11mu}\cos\;\delta} - {S\; 02\mspace{11mu}\sin\;\delta}} \right)}}} & (6)\end{matrix}$

Further, the circular dichroism measuring device according to an aspectof the present invention further includes a beam splitter that splitsthe light emitted from the phase modulation element in two, and a lightdetector constituting the sample data acquisition means and detectinglight transmitted through the sample is formed on an optical path forone of the two lights split by the beam splitter, and the secondpolarization plate constituting the Ip(t) acquisition means and thesecond light detector detecting light transmitted through the secondpolarization plate are formed on an optical path for the other of thetwo lights split by the beam splitter.

In such a circular dichroism measuring device, since it is possible tosimultaneously realize the sample data acquisition means and the Ip(t)acquisition means by forming two optical paths using the beam splitter,it is possible to acquire a phase change of the phase modulation elementin real time. Thus, for example, even when a phase modulation elementwith significant temperature characteristics or drift characteristics,such as a liquid crystal phase modulation element, is used, it ispossible to perform high-accuracy circular dichroism measurement.

Advantageous Effects of Invention

According to the present invention, the circular dichroism measuringmethod and the circular dichroism measuring device capable of accuratelymeasuring circular dichroism even when a phase modulation element with adistortion component is used are provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a schematic configuration of a circulardichroism measuring device of the related art.

FIG. 2 is a diagram illustrating a schematic configuration of a circulardichroism measuring device according to a first embodiment.

FIG. 3 is a diagram illustrating linear dichroism (LD) of a sample.

FIG. 4 is a flow diagram illustrating a circular dichroism measuringmethod.

FIG. 5 is a diagram illustrating a waveform of a synchronization signalfrom a phase modulation element driver.

FIG. 6 illustrates data Ip(t) measured using a circular dichroismmeasuring device using a CaF2 PEM.

FIG. 7 illustrates data δ(t) of a phase change amount in the circulardichroism measuring device using the CaF2 PEM.

FIG. 8 is a diagram illustrating dependence of an S03 value on arotational angle obtained using a method of the related art based onIs(t).

FIG. 9 is a diagram illustrating dependence of an S02 value on arotational angle obtained using a method of the related art based onIs(t).

FIG. 10 is a diagram illustrating a value of S02 measured using a methodof the related art using a PEM-100/IFS50 model made of quartz as a phasemodulation element.

FIG. 11 is a diagram illustrating dependence of an S03 value on arotational angle calculated according to a circular dichroism measuringmethod of this embodiment using a CaF₂ PEM as a phase modulationelement.

FIG. 12 is a diagram illustrating dependence of an S02 value on arotational angle calculated according to a circular dichroism measuringmethod of this embodiment using a CaF₂ PEM as a phase modulationelement.

FIG. 13 is a diagram illustrating a schematic configuration of acircular dichroism measuring device according to a second embodiment.

FIG. 14 is a diagram illustrating a relationship obtained using Equation(23).

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. The same elementsare denoted with the same reference numerals in description of thedrawings, and repeated description will be omitted.

First Embodiment

In the following description, schematic description of a configurationof a circular dichroism measuring device of the related art and circulardichroism thereof will be described and then a circular dichroismmeasuring device and a circular dichroism measuring method according tothe first embodiment will be described. In the following description, atraveling direction of light from a light source is a Z-axis, and twoaxes perpendicular and orthogonal to the Z-axis are an X-axis (verticaldirection) and a Y-axis (horizontal direction).

FIG. 1 is a schematic configuration diagram of a circular dichroismmeasuring device of the related art. As illustrated in FIG. 1, in thecircular dichroism measuring device, a light source 10, a polarizationplate 20, a phase modulation element 30, and a light detector 50 arearranged in this order, and a sample 40 is arranged between the phasemodulation element 30 and the light detector. Further, the modulation ofthe phase modulation element 30 is controlled by a phase modulationelement driver 31, and a synchronization signal from the phasemodulation element driver 31 is sent to a lock-in amplifier 60 connectedto the light detector 50.

The light source 10 emits light for irradiating the sample. The lightemitted from the light source 10 is unpolarized light. For example, adeuterium lamp that emits light with a wavelength of 280 nm is used asthe light source 10.

The light emitted from the light source 10 is incident on thepolarization plate 20. In the polarization plate 20, linearly polarizedlight is extracted from the light emitted from the light source 10. Forexample, a Glan-Taylor prism is used as the polarization plate 20. Here,the polarization plate 20 is assumed to extract linearly polarized lightin a direction of 0° with respect to an X-axis. A filter for removing awavelength component, a noise component, or the like unnecessary forcircular dichroism measurement may be provided at a preceding stage withrespect to the polarization plate 20.

The linearly polarized light extracted by the polarization plate 20 isconverted into right circularly polarized light or left circularlypolarized light by a circular polarization modulator and emitted. Thephase modulation element 30 (phase modulation means) and the phasemodulation element driver 31 as a phase modulation signal oscillator areincluded. The phase modulation element 30 has an optical axis in adirection of 45° around the Z-axis with respect to the X-axis. The phasemodulation element driver 31 periodically alternately oscillates asignal for instructing conversion to the right circularly polarizedlight and a signal for instructing conversion to the left circularlypolarized light, as a modulation signal. The phase modulation elementdriver 31 oscillates, for example, a modulation signal with a period of50 kHz. The linearly polarized light incident on the phase modulationelement 30 is modulated on the basis of the modulation signal oscillatedby the phase modulation element driver 31 so that a phase differencebetween two orthogonal polarized light components periodically changes,to be alternately converted into the right or left circularly polarizedlight and emitted. For example, a photoelastic modulator or a Pockelcell may be used as a specific element for the phase modulation element30.

Further, the modulation signal generated by the phase modulation elementdriver 31 is sent from the phase modulation element driver 31 to thelock-in amplifier 60, as a synchronization signal. In the lock-inamplifier 60, the intensity of the light detected by the light detector50 and the modulation signal from the phase modulation element driver 31are combined such that the measurement of the circular dichroism isperformed.

Here, a specific method of calculating the circular dichroism will bedescribed. The circular dichroism is defined as a difference betweenabsorbance A_(L) with respect to the left circularly polarized light andabsorbance A_(R) with respect to the right circularly polarized light.The measurement of the circular dichroism is realized by irradiating thesample with the left and right circularly polarized lights according tothis definition and obtaining a difference between the absorbances fromthe intensity of transmitted light. This measuring method is expressedin a Mueller matrix method as follows. First, a Mueller matrix accordingto the sample is expressed by Equation (11) below. Here, A_(v) is anaverage absorbance of the sample.

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 7} \right\rbrack & \; \\{S = {{S(\theta)} = {e^{- {Av}} \cdot \begin{pmatrix}{S\; 00} & {S\; 01} & {S\; 02} & {S\; 03} \\{S\; 10} & {S\; 11} & {S\; 12} & {S\; 13} \\{S\; 20} & {S\; 21} & {S\; 22} & {S\; 23} \\{S\; 30} & {S\; 31} & {S\; 32} & {S\; 33}\end{pmatrix}}}} & (11)\end{matrix}$

According to the Mueller matrix method, if the left circularly polarizedlight is transmitted through the sample, an intensity of the transmittedlight is expressed by Equation (12) below.

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 8} \right\rbrack & \; \\{{{e^{- {Av}} \cdot \begin{pmatrix}{S\; 00} & {S\; 01} & {S\; 02} & {S\; 03} \\{S\; 10} & {S\; 11} & {S\; 12} & {S\; 13} \\{S\; 20} & {S\; 21} & {S\; 22} & {S\; 23} \\{S\; 30} & {S\; 31} & {S\; 32} & {S\; 33}\end{pmatrix}}\begin{pmatrix}1 \\0 \\{- 1} \\0\end{pmatrix}} = {e^{- {Av}} \cdot \begin{pmatrix}{{S\; 00} - {S\; 02}} \\{{S\; 10} - {S\; 12}} \\{{S\; 20} - {S\; 22}} \\{{S\; 30} - {S\; 32}}\end{pmatrix}}} & (12)\end{matrix}$

That is, the light intensity obtained as an output is e^(−Av)·(S00−S02).Since an incident light intensity is 1, the absorbance A_(L) iscalculated as in Equation (13) below.

[Math. 9]A _(L)=−log[e ^(−Av)·(S00−S02)]  (13)

Similarly, if the right circularly polarized light is transmittedthrough the sample, the intensity of the transmitted light is expressedby Equation (14) below.

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 10} \right\rbrack & \; \\{{{e^{- {Av}} \cdot \begin{pmatrix}{S\; 00} & {S\; 01} & {S\; 02} & {S\; 03} \\{S\; 10} & {S\; 11} & {S\; 12} & {S\; 13} \\{S\; 20} & {S\; 21} & {S\; 22} & {S\; 23} \\{S\; 30} & {S\; 31} & {S\; 32} & {S\; 33}\end{pmatrix}}\begin{pmatrix}1 \\0 \\1 \\0\end{pmatrix}} = {e^{- {Av}} \cdot \begin{pmatrix}{{S\; 00} + {S\; 02}} \\{{S\; 10} + {S\; 12}} \\{{S\; 20} + {S\; 22}} \\{{S\; 30} + {S\; 32}}\end{pmatrix}}} & (14)\end{matrix}$

That is, the light intensity obtained as an output is e^(−Av)·(S00+S02).Since the incident light intensity is 1, the absorbance A_(R) iscalculated as in Equation (15) below.

[Math. 11]A _(R)=−log[e ^(−Av)·(S00+S02)]  (15)

Since the circular dichroism is calculated from a difference between theleft and right circularly polarized lights, a difference between A_(L)and A_(R) is obtained as in Equation (16) below.

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 12} \right\rbrack & \; \\\begin{matrix}{{A_{L} - A_{R}} = {{{- \log}\left\lfloor {e^{- {Av}} \cdot \left( {{S\; 00} - {S\; 02}} \right)} \right\rfloor} + {\log\left\lfloor {e^{- {Av}} \cdot \left( {{S\; 00} + {S\; 02}} \right)} \right\rfloor}}} \\{= {\log\left\lbrack \frac{e^{- {Av}} \cdot \left( {{S\; 00} + {S\; 02}} \right)}{e^{- {Av}} \cdot \left( {{S\; 00} - {S\; 02}} \right)} \right\rbrack}} \\{= {- {\log\left\lbrack \frac{{S\; 00} - {S\; 02}}{{S\; 00} + {S\; 02}} \right\rbrack}}} \\{= {- {\log\left\lbrack {1 + \frac{{- S}\; 02}{{S\; 00} + {S\; 02}}} \right\rbrack}}}\end{matrix} & (16)\end{matrix}$

Here, since a value of S02 is usually very small compared to S00, afraction in Equation (16) is very small compared to 1. Since, generally,x is sufficiently smaller than 1 and log(1+x) approximates to x, if thisis applied to Equation (16), the difference between the left and rightcircularly polarized lights can be finally expressed as in Equation(17).

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 13} \right\rbrack & \; \\{{A_{L} - A_{R}} = {{- {\log\left\lbrack {1 + \frac{{- S}\; 02}{{S\; 00} + {S\; 02}}} \right\rbrack}} \approx \frac{S\; 02}{{S\; 00} + {S\; 02}} \approx \frac{S\; 02}{S\; 00}}} & (17)\end{matrix}$

That is, measurement of the circular dichroism as in the definitioncorresponds to obtaining a ratio of S02 to S00 in the Mueller matrixaccording to the sample.

Here, the light intensity detected by the light detector 50 of thecircular dichroism measuring device 1 of FIG. 1 is expressed by Equation(18) using a Mueller matrix method.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 14} \right\rbrack & \; \\{I = {\frac{1}{2}{e^{- {Av}} \cdot \left( {{S\; 00} + {S\; 0\; 3\mspace{11mu}\cos\;\delta} - {S\; 02\mspace{11mu}\sin\;\delta}} \right)}}} & (18)\end{matrix}$

Here, δ is a phase change amount of the phase modulation element, andtemporally varies as shown in Equation (19) below.

[Math. 15]δ=δ₀ sin(ωt+α)  (19)

Here, ω denotes a modulation angular frequency (=2 πf: f is a modulationfrequency) of the phase modulation element 30, δ0 denotes a maximumdelay amount of the phase modulation element 30, and α denotes residualdistortion of the phase modulation element 30, linearly polarized lightbirefringence derived from the an optical component, or both of them.

Here, when the circular dichroism measuring device has an idealconfiguration, that is, when the phase modulation element 30 has nodistortion component and linearly polarized light birefringence due tooptical components including others does not occur, α=0. Accordingly,Equation (18) above can be developed as Equation (20). Here, J_(x) is anx-order Bessel function.

$\begin{matrix}{\mspace{79mu}\left\lbrack {{Math}.\mspace{14mu} 16} \right\rbrack} & \; \\{I = {\frac{1}{2}{e^{- {Av}}\left\lbrack {{S\; 00} + {S\; 0\;{3\; \cdot {J_{0}\left( \delta_{0} \right)}}} + {S\;{03 \cdot 2}{J_{2}\left( \delta_{0} \right)}\;{\cos\left( {2\;\omega\; t} \right)}} + \ldots - {S\;{02 \cdot 2}{J_{1}\left( \delta_{0} \right)}\sin\;\left( {\omega\; t} \right)} - \ldots} \right\rbrack}}} & (20)\end{matrix}$

According to Equation (20), S02 appears as a frequency ω component.Further, in Equation (20), S03 appears as a 2ω component. Thus, the S02component and the S03 component can be measured individually byseparating S02 and S03 using the lock-in amplifier or the like.

However, the above is a case in which the circular dichroism measuringdevice has an ideal configuration, and when α is not zero, that is, whenthere is residual distortion in the phase modulation element 30 or thereis linear polarization birefringence in an optical component, Equation(18) is developed as in Equation (21) below.

$\begin{matrix}{\mspace{79mu}\left\lbrack {{Math}.\mspace{14mu} 17} \right\rbrack} & \; \\{I = {\frac{1}{2}{e^{- {Av}}\left\lbrack {{S\; 00} + {S\; 0\;{3\; \cdot \cos}\;{\alpha \cdot {J_{0}\left( \delta_{0} \right)}}} + {S\;{03 \cdot \cos}\;{\alpha \cdot 2}{J_{2}\left( \delta_{0} \right)}\;{\cos\left( {2\;\omega\; t} \right)}} - {S\;{03 \cdot \sin}\;{\alpha \cdot 2}{J_{1}\left( \delta_{0} \right)}\sin\;\left( {\omega\; t} \right)} + \ldots - {S\;{02 \cdot \sin}\;{\alpha \cdot {J_{0}\left( \delta_{0} \right)}}} - {S\;{02 \cdot \cos}\;{\alpha \cdot 2}{J_{1}\left( \delta_{0} \right)}\sin\;\left( {\omega\; t} \right)} + {S\;{02 \cdot \sin}\;{\alpha \cdot 2}\;{J_{2}\left( \delta_{0} \right)}{\cos\left( {2\;\omega\; t} \right)}} + \ldots} \right\rbrack}}} & (21)\end{matrix}$

In Equation (21), it is shown that S03 is incorporated into a frequencyω component and S02 is incorporated into a 2ω component with a weight ofsin α due to residual distortion α, as indicated by an underline inEquation (21), that is, it is not possible to separate S02 and S03through the analysis in a frequency domain of a lock-in amplifier or thelike.

S03 is for linear dichroism (LD) according to a general equation of theMueller matrix of an optically active sample. In a liquid sample inwhich S03=LD=0 is satisfied, the above problem does not occur. However,in a solid, gel, or film-like sample, there is not little LD, and aproblem occurs in that this LD is incorporated into a CD signal as anartifact component.

Therefore, in CD measurement of the related art, since a phasemodulation element with less residual distortion is required andadoption of a high-accuracy phase modulation element such as aphotoelastic modulator (PEM) is essential, the entire circular dichroismmeasuring device has been inevitably expensive.

On the other hand, in the circular dichroism measuring method using thecircular dichroism measuring device according to this embodiment, it ispossible to separate S02 and S03 in the Mueller matrix even when thephase modulation element includes a distortion component by separatelymeasuring the phase change amount with respect to time of the phasemodulation element.

FIG. 2 illustrates a configuration of the circular dichroism measuringdevice 2 according to the first embodiment. As illustrated in FIG. 2,the circular dichroism measuring device 2 includes a light source 10, afirst polarization plate 21 (polarization plate 1), a phase modulationelement 30, a phase modulation element driver 31, a second polarizationplate 22 (polarization plate 2), a light detector 50, an AD conversionboard 70, and a computer 80. The second polarization plate 22 and thesample 40 are held by an exchange mechanism 90 capable of exchanging anobject disposed on the optical path for the light from the light source10, and either of the second polarization plate 22 and the sample 40 isdisposed on the optical path. The first polarization plate 21 is assumedto extract linearly polarized light in a direction of 0° with respect tothe X-axis, similar to the polarization plate 20. Further, the phasemodulation element 30 is similarly set in a direction of 45° around theZ-axis with respect to the X-axis. Further, it is assumed that thedirection of the second polarization plate 22 is changed for eachmeasurement, and linearly polarized light in the changed direction isextracted.

In the above circular dichroism measuring device 2, the light from thelight source 10 is converted into linearly polarized light in the X-axisdirection by the first polarization plate 21 and phase-modulated by thephase modulation element 30 set in a direction of 45° around the Z-axiswith respect to the X-axis, and modulated light including right and leftcircularly polarized lights is generated. In the circular dichroismmeasuring device 2 according to this embodiment, a PEM-100/ICF50 modelwhich is a photoelastic modulator (PEM) available from Hinds Instrumentscan be used as the phase modulation element 30. A material of thephotoelastic modulator (PEM) is CaF2. The phase modulation element 30 isdriven by the phase modulation element driver 31, similar to thecircular dichroism measuring device 1. A synchronization signal formodulation is simultaneously output from the phase modulation elementdriver 31. A modulation frequency may be, for example, 50 kHz.

The modulated light generated by the phase modulation element 30 istransmitted through the sample 40 or the second polarization plate 22held by the exchange mechanism 90, and is detected by the light detector50. A configuration in which exchange of the sample 40 and the secondpolarization plate 22 is manually performed may be adopted in place ofthe configuration in which the exchange mechanism 90 is used. In thefollowing embodiments, a case in which a sample obtained by extending apolyvinyl alcohol film stained with Congo Red dye is used as the sample40 will be described. The polyvinyl alcohol film stained with Congo Reddye has a large linear dichroism (LD), as illustrated in FIG. 3.

A signal according to the light detected in the light detector 50 isconverted into digital data by the AD conversion board 70 inserted intothe computer 80, and then, stored in a first waveform memory 81(waveform memory 1) or a second waveform memory 82 (waveform memory 2)inside the computer 80. A timing of the start of AD conversion in the ADconversion board 70 is determined on the basis of the synchronizationsignal that is transmitted from the phase modulation element driver 31.

In the circular dichroism measuring device 2, the light source 10, thefirst polarization plate 21, the phase modulation element 30, and thelight detector 50 function as an Is(t) acquisition means (sample dataacquisition means) that acquires data Is(t) indicating a temporal changein a light signal transmitted through the sample. Further, the lightsource 10, the first polarization plate 21, the second polarizationplate 22, the phase modulation element 30, and the light detector 50function as an Ip(t) acquisition means (phase amount change acquisitionmeans) that acquires data Ip(t) indicating a temporal change in thelight signal due to the phase modulation element. Further, the computer80 functions as conversion means (analysis means) for converting thedata Ip(t) into data δ(t) indicating the temporal change in the phasechange amount due to the phase modulation element, an Is(δ) calculationmeans (analysis means) for calculating the data Is(δ) according to thephase amount at time t of the data Is(t) of the sample on the basis ofthe data δ(t) of the phase change amount, and a matrix elementcalculation means (analysis means) for performing fitting on the dataIs(δ) and calculating matrix elements S00, S02, and S03 in a Muellermatrix according to the sample.

Further, the circular dichroism measuring method using the circulardichroism measuring device 2 includes steps illustrated in FIG. 4. Thatis, a step of measuring Ip(t) (S101: phase amount change acquisitionstep), a step of measuring Is(t) (S102: sample data acquisition step), astep of converting Ip(t) to δ(t) (S103: phase amount change acquisitionstep), a step of converting Is(t) to Is(δ) (S104: analysis step/Is(δ)calculation step), and a step of performing curve fitting to calculatethe matrix elements S00, S02, and S03 (S105: analysis step/matrixelement calculation step).

An order of the above steps may be appropriately changed. In particular,there is no problem even when an order of the step of measuring Ip(t)(S101) and the step of measuring Is(t) (S102) is switched, and the stepof converting Ip(t) to δ(t) (S103) may be performed after the step ofmeasuring Ip(t) (S101). It is necessary for the step of converting Is(t)to Is(δ) (S104) to be performed after all of preceding steps (S101 toS103) end. The step of performing curve fitting (S105) is performedafter the step of converting Is(t) to Is(δ) (S104).

First, the second polarization plate 22 is disposed in the optical path.The second polarization plate 22 is disposed so that the polarizationaxis has a relationship with the X-axis direction, that is, the firstpolarization plate 21 giving parallel Nicols. In this state, measurementlight is emitted from the light source 10, and a light signal intensityobtained without driving the phase modulation element 30 is recorded.This intensity is referred to as Im.

Then, the second polarization plate 22 is disposed so that thepolarization axis has a relationship with the Y-axis direction, that is,the first polarization plate 21 giving crossed Nicols. In this state,measurement light is emitted from the light source 10, and a lightsignal detected by the light detector 50 in a state in which the phasemodulation element 30 is driven is stored in the first waveform memory81. This stored data is referred to as Ip(t) (S101).

Then, the sample 40 is installed in the optical path in place of thesecond polarization plate 22 by the exchange mechanism 90. In thisstate, measurement light is emitted from the light source 10, and thelight signal detected by the light detector 50 in a state in which thephase modulation element 30 is driven is stored in the second waveformmemory 82. The stored data is referred to as Is(t) (S102).

Here, the data Ip(t) recorded in step S01 is converted into data δ(t) ofthe phase change amount with respect to time using Equation (22) below.

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 18} \right\rbrack & \; \\{{\delta(t)} = {\cos^{- 1}\left( {1 - \frac{2{I_{P}(t)}}{I_{m}}} \right)}} & (22)\end{matrix}$

According to Equation (22) above, it is seen only that the sign of δvaries according to time. Specifically, since the phase change amountvaries according to a synchronization signal from the phase modulationelement driver 31, positive or negative is discriminated between on thebasis of a sign of the synchronization signal.

A specific method of calculating δ will be described with reference toFIGS. 5 and 6. FIG. 5 is a diagram illustrating a waveform of anactually measured synchronous signal from the phase modulation elementdriver 31. Further, FIG. 6 illustrates data Ip(t) measured using thecircular dichroism measuring device 2 that uses a CaF2 PEM as the phasemodulation element 30.

Here, an absolute value of δ can be obtained by adapting Equation (12)described above to a value at each time in FIG. 6. For a sign of δ, inFIG. 5, δ is positive in a time domain in which the synchronizationsignal is at a high level (with intensity 5), and δ is negative in atime domain in which the synchronization signal is at a low level (withless than intensity 0). Data δ(t) of the phase change amount in thecircular dichroism measuring device 2 using a CaF₂ PEM obtained actuallyin the above operation is illustrated in FIG. 7 (S103).

A phase amount at a time t of the measurement data Is(t) of the sample,that is, Is(δ) is obtained from the data δ(t) of the phase change amountillustrated in FIG. 7 (S104). Then, it is possible to directly obtainS00, S02, and S03 in the Mueller matrix according to the sample byperforming curve fitting on a relationship of Is(δ) using the functionshown in Equation (18) above (S105).

Hereinafter, effects obtained by using the circular dichroism measuringmethod according to this embodiment will be described while comparingwith a method of the related art.

The sample 40 was rotated around the Z-axis from 0° to 90° with anincrement of 15°, and dependence of S02 and S03 on an angle of rotationwas evaluated. S03 that is a linear dichroism (LD) term greatly dependson the rotational angle, but if the circular dichroism measuring devicehas an ideal configuration, S02 is estimated not to depend on therotational angle.

The dependence of the S03 value on the rotational angle obtained using amethod of the related art, that is, a method of extracting an angularfrequency component through FFT from Is(t) using a CaF₂ PEM as the phasemodulation element 30, similar to the above embodiment, is illustratedin FIG. 8. Further, the dependence of the S02 value on the rotationalangle is illustrated in FIG. 9. As illustrated in FIG. 7, in the CaF₂PEM, it can be seen that the phase change amount does not exactly reach90° and the CaF2 PEM used in this embodiment has residual distortion. Inthe case of such a PEM, S03, that is, a LD component is incorporatedinto S02, and great dependence on the rotational angle appears, as shownin Equation (21). In FIG. 9 illustrating actually measured S02, greatdependence on the rotational angle is also illustrated. As describedabove, when the circular dichroism measuring device has an idealconfiguration, it can be seen that S02 is greatly influenced by adistortion component of the PEM since S02 does not depend on therotational angle.

FIG. 10 illustrates a value of S02 measured by a method of the relatedart using a PEM-100/IFS50 model made of quartz as the phase modulationelement 30. Since the PEM made of quartz has substantially no residualdistortion, even when a circular dichroism measuring method according tothe method of the related art is applied, there is no problem ofresidual distortion. Actually, FIG. 10 illustrates that rotational angledependence of S02 is very small.

Next, a case in which the circular dichroism measuring method accordingto an embodiment of the present invention is used will be described.Rotational angle dependence of an S03 value calculated according to thecircular dichroism measuring method of this embodiment illustrated inFIG. 4 using a CaF₂ PEM as the phase modulation element 30 isillustrated in FIG. 11. Similarly, the rotational angle dependence ofthe S02 value is illustrated in FIG. 12. According to FIGS. 11 and 12,the rotational angle dependence can be confirmed only in S03 (FIG. 11),and an influence of S03, that is, a linear dichroism (LD) component isnot observed in S02 (FIG. 12).

The above result shows that the same data as in a quartz PEM having noresidual distortion, that is, the measurement data not greatlyinfluenced by the distortion component is obtained from the CaF₂ PEMhaving residual distortion by applying the circular dichroism measuringmethod according to the present invention.

A phase difference amount is obtained from the obtained light intensityby inserting the PEM (phase modulation element 30) between thepolarization plates (the first polarization plate 21 and the secondpolarization plate 22) set in the crossed Nicols described above in theabove embodiment, but this does not preclude further use of an advancedphase difference measuring method.

Second Embodiment

Next, a second embodiment will be described using a device illustratedin FIG. 13. Repeated description of portions described in the firstembodiment will be avoided and different portions will be described indetail.

The circular dichroism measuring device 2 shown in the first embodimentperforms a plurality of measurements in order to obtain Im, Is(t), andIp(t) required to obtain Is(δ), whereas a circular dichroism measuringdevice 3 illustrated in FIG. 13 has a configuration aiming at acquiringinformation necessary for measurement of circular dichroism through onemeasurement. That is, Is(t) acquisition means and Ip(t) acquisitionmeans are simultaneously realized using a beam splitter.

Specifically, in the circular dichroism measuring device 3 illustratedin FIG. 13, a first beam splitter 11 is disposed between the phasemodulation element 30 and the sample 40 to split light in two before thesample 40 is irradiated with light transmitted through the phasemodulation element 30. One of the split lights is transmitted throughthe sample 40, and then, converted into an electrical signal by a lightdetector 50. The other of the split lights is guided to a second beamsplitter 12 and split into two lights.

One of the lights split in the second beam splitter 12 is transmittedthrough a third polarization plate 23 (polarization plate 3) andconverted into an electrical signal by a second light detector 51 (lightdetector 2). In this case, the third polarization plate 23 is disposedso that a polarization axis thereof has a relationship with a Y-axis,that is, the first polarization plate 21 giving crossed Nicols. Further,the other of the lights split in the second beam splitter 12 istransmitted through a ¼λ wavelength plate 33 and a fourth polarizationplate 24 (polarization plate 4), and then, converted into an electricalsignal by a third light detector 52 (light detector 3). The fourthpolarization plate 24 is disposed so that a polarization axis thereofhas a relationship with the X-axis, that is, the first polarizationplate 21 giving parallel Nicols. Further, a fast axis of the ¼λwavelength plate 33 is set in a direction of 45° around a Z-axis withrespect to the polarization axis of the first polarization plate 21.

As the first beam splitter 11 and the second beam splitter 12, anon-polarization type beam splitter should be used so that a change in apolarization state does not occur at the time of light splitting.

In the circular dichroism measuring device 3 having the aboveconfiguration, optical components provided on each optical path formedbetween the light source and the light detector are as follows. A numberin parentheses indicates a rotational angle around the Z-axis from theX-axis.

(Optical path A) light source 10→first polarization plate 21 (0)→sample40→light detector 50

(Optical path B) light source 10→first polarization plate 21 (0)→thirdpolarization plate 23 (90)→second light detector 51

(Optical path C) light source 10→first polarization plate 21 (0)→¼λwavelength plate 33 (45)→fourth polarization plate 24 (0)→third lightdetector 52

The electrical signal due to the light detected by the light detector50, the second light detector 51, and the third light detector 52 isinput to the AD conversion board 70 attached to the computer 80,converted into a digital signal, and then, assigned to and stored in thefirst waveform memory 81 and the second waveform memory 82. In thiscase, a relationship between the optical path and data obtained in thedetector on the optical path is as follows.

(1) Using the optical path A, Is(t) is obtained from the signal obtainedby the light detector 50 (that is, the optical path A is Is(t)acquisition means).

(2) Using the optical path B, Ip(t) is obtained from the signal obtainedby the second light detector 51 (that is, the optical path B is Ip(t)acquisition means).

(3) Using the optical path C, Ir(t) is obtained from the signal obtainedby the third light detector 52.

Here, for Ir, if the light intensity measured in the third lightdetector 52 is analyzed using a Mueller matrix method, Equation (23)below is obtained. A relationship obtained in Equation (23) isillustrated in FIG. 14.

$\begin{matrix}\left\lbrack {{Math}.\mspace{11mu} 19} \right\rbrack & \; \\{\frac{{Ir}(t)}{I_{m}} = {\frac{1}{2}\left\{ {1 - {\sin\left( {{\omega\; t} + \alpha} \right)}} \right\}}} & (23)\end{matrix}$

According to FIG. 14, when Ir/Im is equal to or greater than 0.5, aretardation amount is negative, and when Ir/Im is equal to or smallerthan 0.5, the retardation amount is positive. Therefore, Equation (22)is applied to Ip(t) to calculate an absolute value of δ(t), and a signof δ(t) is determined according to whether Ir(t)/Im is smaller orgreater than 0.5. Accordingly, the data δ(t) of the phase change amountcan be obtained.

It is possible to obtain the phase amount at a time t of the measurementvalue Is(t) of the sample, that is, Is(δ) on the basis of the data δ(t)of the phase change amount obtained as above and Is(t) obtained from themeasurement in the optical path A, similar to the first embodiment. S00,S02, and S03 in the Mueller matrix are directly obtained by performingcurve fitting on a relationship of Is(δ) using the function shown inEquation (18).

Thus, when the circular dichroism measuring device 3 according to thesecond embodiment is used, it is possible to perform the circulardichroism measurement using the same steps S103 to S105 as in thecircular dichroism measuring method according to the first embodimentillustrated in FIG. 4. For steps S101 and S102, it is possible to usethe measurement based on the optical path A and the measurement based onthe optical path B in the circular dichroism measuring device 3according to the second embodiment.

That is, in the circular dichroism measuring device 3 according to thesecond embodiment, the replacement between the sample 40 and the secondpolarization plate 22 as in the circular dichroism measuring device 2according to the first embodiment is not necessary, and it is possibleto acquire the retardation (phase difference) of the phase modulationelement in real time. Thus, for example, even when a phase modulationelement with significant temperature characteristics or driftcharacteristics, such as a liquid crystal phase modulation element, isused, it is possible to obtain optical constants such as S00, S02, andS03 with good accuracy. Accordingly, it is possible to performhigh-accuracy circular dichroism measurement.

The embodiments of the present invention have been described above, butthe present invention is not limited to the above embodiments andvarious changes can be performed.

REFERENCE SIGNS LIST

-   1 to 3: Circular dichroism measuring device-   10: Light source-   20 to 24: Polarization plate-   30: Phase modulation element-   31: Phase modulation element driver-   40: Sample-   50 to 52: Light detector-   60: Lock-in amplifier-   70: AD conversion board-   80: Computer-   81, 82: Waveform memory

The invention claimed is:
 1. A circular dichroism measuring deviceincluding a light source, a polarization plate that extracts linearlypolarized light from light emitted from the light source, a phasemodulation element that modulates the linearly polarized light, and alight detector that detects light modulated by the phase modulationelement and then transmitted through a sample, the circular dichroismmeasuring device comprising: a sample data acquisition means foracquiring a change in a light intensity with respect to time in thelight detector; a phase amount change acquisition means for acquiring achange in a phase amount with respect to time of the phase modulationelement; and at least one processor having a memory, the at least oneprocessor configured to perform operations comprising: converting thechange in light intensity acquired in the sample data acquisition meansinto a change with respect to the phase amount on the basis of thechange in the phase amount acquired in the phase amount changeacquisition means, and calculating matrix elements S00, S02, and S03when a Mueller matrix according to the sample is as shown in Equation(1) below on the basis of the change with respect to the phase amount:$\begin{matrix}{S = {{S(\theta)} = {e^{- {Av}} \cdot {\begin{pmatrix}{S\; 00} & {S\; 01} & {S\; 02} & {S\; 03} \\{S\; 10} & {S\; 11} & {S\; 12} & {S\; 13} \\{S\; 20} & {S\; 21} & {S\; 22} & {S\; 23} \\{S\; 30} & {S\; 31} & {S\; 32} & {S\; 33}\end{pmatrix}.}}}} & (1)\end{matrix}$
 2. The circular dichroism measuring device according toclaim 1, wherein the sample data acquisition means acquires data Is(t)indicating a temporal change in a light signal detected in the lightdetector by emitting light from the light source in a state in which thesample is disposed, and the phase amount change acquisition meansincludes: an Ip(t) acquisition means for acquiring data Ip(t) indicatinga temporal change in a light signal detected in the light detector byemitting light from the light source in a state in which a secondpolarization plate having a relationship with the polarization plategiving crossed Nicols is disposed on an optical path from the lightsource, in place of the sample; and the at least one processor isconfigured to perform further operations comprising: converting the dataIp(t) into data δ(t) indicating a temporal change of the phase changeamount due to the phase modulation element using Equation (2) below,$\begin{matrix}{{{\delta(t)} = {\cos^{- 1}\left( {1 - \frac{2{I_{P}(t)}}{I_{m}}} \right)}},} & (2)\end{matrix}$ calculating data Is(δ) according to the phase amount attime t of data Is(t) of the sample on the basis of the data δ(t) of thephase change amount, and performing fitting on the data Is(δ) usingEquation (3) below to calculate matrix elements S00, S02, and S03 in theMueller matrix according to the sample: $\begin{matrix}{I = {\frac{1}{2}{e^{- {Av}} \cdot {\left( {{S\; 00} + {S\; 0\; 3\mspace{11mu}\cos\;\delta} - {S\; 02\mspace{11mu}\sin\;\delta}} \right).}}}} & (3)\end{matrix}$
 3. The circular dichroism measuring device according toclaim 2, further comprising: a beam splitter that splits the lightemitted from the phase modulation element in two, wherein a lightdetector constituting the sample data acquisition means and detectinglight transmitted through the sample is formed on an optical path forone of the two lights split by the beam splitter, and the secondpolarization plate constituting the Ip(t) acquisition means and a secondlight detector detecting light transmitted through the secondpolarization plate are formed on an optical path for the other of thetwo lights split by the beam splitter.